Steel sheet for shielding magnetic field and method for manufacturing same

ABSTRACT

The present invention relates to a steel sheet for shielding a magnetic field, which is used for a medical magnetic resonance imaging (MRI) room wall body and the like, and a method for manufacturing the same.

TECHNICAL FIELD

The present disclosure relates to a steel sheet for shielding a magneticfield used in a medical magnetic resonance imaging (MRI) room wall, orthe like, and a method for manufacturing the same.

BACKGROUND ART

A magnetic resonance imaging (MRI) device mainly used in hospitals is adevice applying radio waves to a human body located within a strongmagnetic field and measuring the reflected magnetic field to observebrains, internal organs, and other internal organs. The MRI deviceutilizes a superconducting magnet to generate a static magnetic field,and there have recently been high magnetic field MRI devices of 7 T (T:Tesla) or higher developed to obtain high-resolution images.

When a leakage magnetic field occurs outside an MRI room due to a highmagnetic field generated from the MRI device, malfunctioning ofelectronic equipment is caused by disturbing a signal system of ahigh-precision electronic device. Further, as there have been studiesundertaken to show a high magnetic field has a detrimental effect on thehuman body, it is necessary to use a magnetic shielding material toprevent such high magnetic field from leaking to the outside. Morepreferably, a magnetic shielding material, which has high performanceand is economical, is required. In this regard, it is essential toselect an optimal shielding material and thickness such that a magneticflux density outside a conventional MRI shielding room is less than 5Gauss (=0.5 mT).

In particular, permeability characteristics indicating a high magneticflux density according to a specific magnetic field intensity in amaterial are important in shielding a DC magnetic field, and it isnecessary to have a low coercive force. In order to decrease coerciveforce while increasing permeability, precipitates such as carbon,nitrogen, carbides or nitrides in steel, which hinder movements of amagnetic domain wall, should be minimized, and grain growth should bepromoted while reducing grain boundaries. An addition of impurities tosteel forms finely dispersed precipitates during solidification orduring a heat treatment process after rolling, thereby forming a newmagnetic domain which reduces static magnetic energy and intervenesmovements of the magnetic domain wall. In addition, when impuritiesenter iron as an intrusion type rather than a substitution type, agreater adverse effect is imposed on magnetic characteristics. Theintrusive element reduces permeability by elastically deforming alattice of a solid solution, and carbon is a representative element.

Patent Document 1 has proposed a hot-rolled pickled thick steel sheethaving excellent magnetic field shielding characteristics; however, thesteel sheet of Patent Document 1 is different from that of the presentdisclosure in that it lacks magnetic shielding performance becausemagnetic characteristics are only provided by hot-rolling, coiling andpickling without a heat treatment and is a material for shielding an ACmagnetic fields. Recently, there is increasing demand for steelmaterials having excellent magnetic shielding ability capable ofshielding a DC magnetic field such as an MRI room. (Patent Document 1)Korea Patent Publication No. 10-2005-0129244

DISCLOSURE Technical Problem

An aspect of the present disclosure is to provide a steel sheet havingexcellent magnetic field shielding performance by controlling alloyingelements and heat treatment conditions, and a method for manufacturingthe same.

A technical problem of the present disclosure is not limited thereto,and other problems not mentioned will be clearly understood by thoseskilled in the art from the following description.

Technical Solution

An aspect of the present disclosure relates to a steel sheet forshielding magnetic field, comprising, by weight %, 0.02% or less ofcarbon (C), 0.001% to 0.05% of silicon (Si), 0.01% to 0.2% of manganese(Mn), 0.001% or 0.05% of aluminum (Al), 0.005% or less niobium (Nb),0.07% or less of titanium (Ti), 0.01% or less of nitrogen (N), 0.015% orless of phosphorus (P), 0.005% or less of sulfur (S), and a remainder ofiron (Fe) and inevitable impurities, wherein a microstructure hasferrite as main structure, wherein the ferrite has an average grain sizeof 100 μm or greater.

An another aspect relates to a method for manufacturing a steel sheetfor shielding magnetic field, including reheating a steel slabcomprising by weight %, 0.02% or less of carbon (C), 0.001% to 0.05% ofsilicon (Si), 0.01% to 0.2% of manganese (Mn), 0.001% or 0.05% ofaluminum (Al), 0.005% or less niobium (Nb), 0.07% or less of titanium(Ti), 0.01% or less of nitrogen (N), 0.015% or less of phosphorus (P),0.005% or less of sulfur (S), and a remainder of iron (Fe) andinevitable impurities at a temperature of Ac3 to 1250° C., hot rollingthe reheated slab and finish hot rolling at a temperature of Ar3 orhigher to obtain a steel sheet; and air cooling the steel sheet to roomtemperature.

Advantageous Effects

The present disclosure can provide a steel sheet having excellentmagnetic field shielding performance, capable of suppressing an impacton the human body and external electronic devices when applying a DCmagnetic field and applying the DC magnetic field from an MRI device tooutside of an MRI room by enabling smooth movements of magnetic domains.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating magnetic flux density according tomagnetic field intensity of Inventive Examples 2, 5 and 7 andComparative Example 2.

FIG. 2 is a graph illustrating relative permeability according to themagnetic flux density of Inventive Examples 2, 5 and 7 and ComparativeExample 2.

BEST MODE FOR INVENTION

The present inventors conducted extensive research on the abovetechnical problems, and as a result, controlled alloying elements and amanufacturing process to improve shielding performance of a DC magneticfield to coarse grains and facilitate smooth movements of a magneticdomain when the DC magnetic field is applied, thereby providing a steelmaterial having a size of 15 mmt capable of preventing the DC magneticfield released from hospital MRI devices being applied to the outside anMRI room and affecting the human body and external electronic devices.

First, a composition range of the steel sheet of the present disclosureis described. The steel sheet of the present disclosure may contain, byweight % (hereinafter, “%”), 0.02% or less of carbon (C), 0.001% to0.05% of silicon (Si), 0.01% to 0.2% of manganese (Mn), 0.001% or 0.05%of aluminum (Al), 0.005% or less of niobium (Nb), 0.07% or less oftitanium (Ti), 0.01% or less of nitrogen (N), 0.015% or less ofphosphorus (P), 0.005% or less of sulfur (S), and a remainder of iron(Fe) and inevitable impurities.

C: 0.02% or Less

Carbon (C) significantly reduces permeability by elastically deforming alattice of a solid solution. In addition, as ferrite or carbide formeddue to C interferes a movement of a magnetic domain wall so thatincreases an iron loss, it is preferable not to contain carbon as littleas possible. Accordingly, C is preferably contained of 0.02% or less,more preferably 0.005% or less.

Si: 0.001% to 0.05%

Silicon (Si) is mainly used as a deoxidizer. When Si is contained,solubility of C is reduced, thereby lowering magnetic field shieldingcharacteristics. In this regard, it is preferable to contain 0.05% orless. However, Si less than 0.001% results in insufficientdeoxidization. Accordingly, an amount of Si is preferably 0.001% to0.05%, more preferably 0.001% to 0.05%.

Mn: 0.01% to 0.2%

Manganese (Mn) is an element binding to S to form MnS, which is a factoritself increasing brittleness. In order to reduce the brittleness due toS, it is preferable to contain 0.01% or more. However, since MnS formedat a grain boundary during high temperature heat treatment suppressgrain coarsening, it is preferable that an amount of Mn not exceed 0.2%.

Al: 0.001% to 0.05%

Aluminum (Al) is an element for deoxidizing molten steel inexpensively.To sufficiently obtain such an effect, it is preferable to contain0.001% or more. In the case of an amount exceeding 0.05%, however, Albinds to N to form AlN, thereby suppressing grain coarsening. In thisregard, it is preferable to contain Al in an amount of 0.05% or less.

Nb: 0.005% or Less

Niobium (Nb) is an element precipitated in the form of NbC or Nb(C,N) tosignificantly improve strength of a base material and a weld zone.Further, Nb employed during reheating at a high temperature suppressesrecrystallization of austenite and transformation of ferrite or bainitethereby refine a structure. In this regard, Nb is an element added tosecure strength of a conventional hot rolled steel sheet. Meanwhile, Nbhas an adverse effect on magnetic shielding characteristics due to grainrefinement, and thus, it is preferable that 0.005% or less of Nb becontained.

Ti: 0.07% or Less

Titanium (Ti) has an effect of inhibiting grain growth as a result ofreacting mainly with nitrogen when heated and thus is preferable to beadded to a conventional carbon steel to improve strength and toughness.In the present disclosure, however, not only strength and toughness arenot important factors but also Ti is a detrimental element ingraincoarsening, it is preferable to contain Ti in an amount of 0.07% orless.

N: 0.01% (100 ppm) or Less

Nitrogen (N) is an element forming TiN when simultaneously added with Tiand forming AlN by binding to Al when Ti is not added. In the case inwhich TiN, AlN, or the like, is formed at a grain boundary or within agrain, grain coarsening is suppressed during heat treatment at a hightemperature.

Accordingly, N is preferably contained in an amount of 0.01% (100 ppm)or less, more preferably 0.005% (50 ppm) or less.

P: 0.015% or Less

Phosphorus (P) is an element advantageous in improving strength andcorrosion resistance but may significantly increase brittleness of amaterial. In this regard, it is desirable that an amount thereof bemanaged to be as low as possible. Accordingly, it is preferable that theamount of P be 0.015% or less.

S: 0.005% or Less

Sulfur (S) is an element, which significantly increases brittleness byforming MnS, or the like, and thus is desirable to manage an amount aslow as possible. Accordingly, it is preferable that an amount of S be0.005% or less.

The steel sheet of the present disclosure contains iron (Fe) in additionto the above mentioned alloy elements. However, undesired impurities maybe inevitably incorporated from an environment or a raw material duringa conventional manufacturing process and thus cannot be excluded. Suchimpurities are known to any one of ordinary skill in the art, and willthus not be mentioned in detail.

The steel sheet of the present disclosure has a main structure offerrite and preferably contains 95 area % or more of ferrite, morepreferably 99 area % or more.

It is preferable that an average grain size of the ferrite be 100 μm orgreater. In the case in which the grain size is less than 100 μm afterhot rolling, air cooling, normalizing heat treatment, and stress reliefannealing, a movement of a magnetic domain is not smooth so that amagnetic flux density is not sufficiently high when a magnetic field isinduced on a shielding steel sheet. Thereby, a shielding effect is notsufficient when a DC magnetic field is applied to an MRI of 7 T orhigher.

Meanwhile, the steel sheet of the present disclosure may containprecipitates at a grain boundary or within a grain of the ferrite. Theprecipitates may be AlN, TiN, MnS, or the like. As the precipitatesserve to suppress grain growth due to a pinning effect during thenormalizing heat treatment after rolling, it is preferable that amaximum size of the precipitate not exceed 100 nm, more preferably 20nm.

It is preferable that the steel sheet has yield strength of 190 MPa orless, tensile strength of 220 MPa to 300 MPa, elongation of 40% or more,and a yield ratio of 0.8 or less.

In the case of high magnetic field MRI equipment of 7 T or higher,magnetic field intensity of a DC magnetic field induced on a shieldingsteel sheet forms an MRI room wall body is conventionally 200 A/m to 300A/m and is commonly designed to be applied with a magnetic flux densityof 1.3 T to 1.5 T. Accordingly, the steel sheet of the preventdisclosure is preferably has a magnetic flux density of 1 T (Tesla) orhigher under magnetic field intensity of 300 A/m, more preferably 1.3 Tor higher for better DC magnetic field shielding performance.

A method for manufacturing the steel sheet of the present disclosurewill be described in detail. The steel sheet of the present disclosuremay be manufactured by heating a steel material (a steel slab)satisfying the alloy composition described above and hot rolling. Ifnecessary, stress relief annealing may be performed, in addition to anormalizing heat treatment. Each process will be described in detail.

As an example, a steel material satisfying the previously describedalloy composition, a steel slab, is prepared and heated. The heating ispreferably performed in the temperature range of Ac3 to 1250° C. The Ac3can be calculated using Equation 1 below. When the heating temperatureof the steel slab is less than Ac3, transformation of austenite toferrite is initiated during rolling, thereby adversely affectingmagnetic characteristics due to refinement of grain size of ferrite.Meanwhile, it is preferable that the heating be performed at atemperature not exceeding 1250° C. in consideration of economicalfeasibility.

Ac3(° C.)=937.2−436.5C+56Si−19.7Mn+136.3Ti−19.1Nb+198.4Al  [Equation 1]

Each component symbol refers to a content thereof (wt %).

The heated steel material is hot rolled. The hot rolling processspecifically involves rough rolling of the reheated slab and finishrolling to obtain a hot rolled steel sheet. The rough rolling ispreferably performed in the temperature range of Tnr to 1250° C. Whenthe slab is rolled at a temperature below Tnr, grains are refined,thereby making it ineffective in improving magnetic shieldingperformance. Meanwhile, the Tnr can be derived from Equation 2 below.

Tnr(° C.)=887+464C+(6445Nb−/644Nb)+890Ti+363Al−357Si  [Equation 2]

Each component symbol refers to a content thereof (wt %).

Meanwhile, when the finish rolling is performed in a temperature rangebelow Ar3, at which the austenite begins to transform to ferrite,refined ferrite forms nuclei at a ferrite grain boundary, therebyreducing an average grain size so that adversely affecting magneticcharacteristics. Accordingly, it is preferable that the finish rollingbe performed at a temperature of Ar3 or higher.

Hot rolled steel sheet is then cooled. The cooling is not particularlymanaged. As a preferred example, air cooling is performed, the aircooling is performed until the temperature reaches room temperature.

The cooled steel sheet is normalizing heat treated of maintaining at atemperature of Ac3 or higher for (1.3t+30) minutes, and then, furnacecooling or air cooling. The normalizing heat treatment performed for along period of time at a temperature of Ar3 or higher is effective inimproving magnetic shielding characteristics due to additional graincoarsening. It requires at least (1.3t, t: thickness) minutes to allowan thick steel plate to reach a target temperature, and at least 30minutes of maintaining after the target temperature is reached todistribute a uniform temperature from a surface of the steel sheet to acenter thereof.

To remove stress inside the steel sheet which has been normalizing heattreated, stress relief annealing of heat treating at 800° C. to 900° C.for (1.3t+30) minutes, and then, furnace cooling may be performed.Stress remaining in the hot rolled steel sheet, which has been roughrolled and finish rolled, may interfere movements of the magnetic domainand significantly reduce the magnetic characteristics. In this regard,stress relief annealing of heat treating at 800° C. to 900° C. for(1.3t+30, t: thickness of steel sheet) minutes after the normalizingheat treatment, and then air cooling or furnace cooling, may beadditionally performed to maximize the magnetic shieldingcharacteristics.

MODE FOR INVENTION

The prevent disclosure will be described in more detail with referenceto the Examples. The Examples, however, are merely for understanding ofthe present disclosure and should not be construed as limiting thepresent disclosure. The scope of the present disclosure is determined bysubject matter described in the claims and reasonably inferredtherefrom.

Examples

A steel slab having a thickness of 300 mm and the composition of Table 1is prepared, and a sheet material is manufactured under the condition ofTable 2 below. A unit of the alloy composition of Table 1 is wt %, and aremainder of iron (Fe) and inevitable impurities are included.

TABLE 1 classification C Si Mn P S T-Al Nb Ti N Steel 1 0.0015 0.0020.053 0.0070 0.0038 0.0322 0.0014 0.0686 0.0022 Steel 2 0.0195 0.0010.195 0.0072 0.0032 0.0230 0.0001 0.0001 0.0018 Steel 3 0.0016 0.0030.032 0.0057 0.0045 0.0076 0.0005 0.0003 0.0019 Steel 4 0.1500 0.0100.500 0.0150 0.0150 0.0400 0.0001 0.0001 — Steel 5 0.0300 0.010 0.2000.0100 0.0100 0.035  0.0001 0.0001 — Steel 6 0.0015 0.002 0.069 0.00920.0043 0.035  0.0001 0.0289 0.0095

TABLE 2 Normalizing Normalizing Cooling Stress Slab Finish heat heatafter relief Stress heating rolling treatment treatment Normalizingannealing relief temp temp Thickness temp time heat temp time Steel (°C.) (° C.) (mm) (° C.) (min) treatment (° C.) (min) note Steel 1 1180980 25 950 62.5 air — — IE 1 cooling Steel 1 1180 980 25 950 62.5furnace — — IE 2 cooling Steel 1 1180 980 25 — — — — — IE 3 Steel 2 1180980 25 950 62.5 air — — IE 4 cooling Steel 2 1180 980 25 950 62.5furnace — — IE 5 cooling Steel 3 1140 920 25 910 100 air — — IE 6cooling Steel 3 1140 920 25 910 100 air 840 120 IE 7 cooling Steel 41180 860 25 — — — — — CE 1 Steel 5 1180 890 20 — — — — — CE 2 Steel 61180 920 2.5 — — — — — CE 3 *IE: Inventive Example **CE: ComparativeExample

For the steel sheet manufactured under the conditions of Table 2, agrain size and mechanical properties were evaluated and indicated inTable 3 below. The grain size was observed in a thickness direction ofthe steel sheet using an optical microscope. Meanwhile, the mechanicalproperties, such as yield strength, tensile strength, elongation, andthe like, were evaluated using a tensile tester by taking a fullthickness sample in a rolling direction at room temperature.

TABLE 3 Yield Tensile Grain size strength strength Elongationclassification (μm) (MPa) (MPa) (%) Yield ratio IE 1 357.0 102 258 76.80.40 IE 2 386.0 86 253 77.3 0.34 IE 3 121.5 113 263 76.4 0.43 IE 4 320.5180 289 65.8 0.63 IE 5 345.5 172 281 70.2 0.61 IE 6 137.1 149 270 66.00.62 IE 7 168.3 120 265 71.5 0.45 CE 1 21.1 232 409 42.1 0.57 CE 2 22.4194 304 44.1 0.61 CE 3 35.2 204 272 40.2 0.75 * IE: Inventive Example **CE: Comparative Example

As shown in Table 3, Inventive Examples satisfying the alloy compositionand manufacturing process of the present disclosure have a grain size of100 μm or greater, yield strength of 190 MPa or less, tensile strengthof 220 MPa to 300 MPa, elongation of 40% or more, and a yield ratio of0.8 or less.

Meanwhile, in the case of Inventive Example 3, the same composition asand similar manufacturing processes of Inventive Examples 1 and 2 areapplied, however, a normalizing heat treatment was not performed.Thereby, the grain size of Inventive Example 3 is relatively small ascompared to Inventive Examples 1 and 2.

Meanwhile, Comparative Examples 1 and 2 employs a steel containing C inan amount exceeding the amount suggested in the present disclosure, sothat they contain grains having a refined size as compared to those ofInventive Examples 1 to 7, and thus have yield strength and tensilestrength beyond the ranges suggested in the present disclosure due tograin refinement. Comparative Example 3 has a composition satisfying therange suggested in the present disclosure, however, a thickness of afinal product after hot rolling the slab is small (a total reductionamount is increased), which is beyond the strength range suggested inthe present disclosure.

Meanwhile, a magnetic flux density of the manufactured steel sheet wasmeasured with respect to magnetic field intensity and indicated in Table4 below.

TABLE 4 Magnetic flux density to magnetic field intensity (T) B2 (200 B3(300 B5 (500 B25 (2500 classification A/m) A/m) A/m) A/m) IE 1 1.14 1.361.50 1.66 IE 2 1.33 1.44 1.51 1.65 IE 3 0.88 1.11 1.34 1.65 IE 4 0.741.04 1.32 1.64 IE 5 0.78 1.16 1.44 1.66 IE 6 1.03 1.23 1.38 1.64 IE 71.41 1.48 1.54 1.65 CE 1 0.02 0.11 0.36 1.55 CE 2 0.22 0.52 0.86 1.63 CE3 0.74 0.98 1.34 1.59 * IE: Inventive Example ** CE: Comparative Example

As indicated in Table 4 above, Inventive Examples 1 to 7 satisfying thealloy composition and manufacturing method of the present disclosurehave a magnetic flux density of 1.0 T or higher at magnetic fieldintensity of B3.

Meanwhile, Inventive Example 3, among all Inventive Examples, does notinvolve additional normalizing heat treatment after hot rolling andcooling and showed relative lower values as compared to InventiveExamples 1 and 2, which are in similar conditions. In contrast,Inventive Example 7 is a result of performing stress relief annealingand thus has a highest excellent magnetic flux density compared to theother conditions.

Meanwhile, Comparative Examples 1 to 3 have a magnetic flux density of0.11 T to 0.98 T at magnetic field intensity of B3, indicatinginappropriateness to be used as a material for an MRI shielding room of7 T or higher.

FIG. 1 is a graph illustrating magnetic flux density according tomagnetic field intensity of Inventive Examples 2, 5 and 7 andComparative Example 2. In a certain magnetic field intensity region, themagnetic flux density was found to be high in the order of Examples 7,2, and 5, while Comparative Example 2 showed a lowest magnetic fluxdensity. It can be seen that the magnetic characteristics are thehighest when the stress relief annealing and normalizing heat treatmentare performed after hot rolling and the composition and componentssuggested in the present disclosure are satisfied.

FIG. 2 is a graph illustrating relative permeability according to themagnetic flux density of Inventive Examples 2, 5 and 7 and ComparativeExample 2. Permeability is a value indicating a degree of magnetizationof a medium in a given magnetic field, while relative permeabilityindicates a ratio of permeability of a medium to vacuum permeability andis represented as the equation “magnetic flux density/magnetic fieldintensity/1.257.” The relative permeability of FIG. 2 illustrates aresult of each sample indicated in FIG. 1 calculated using the equationwith respect to the magnetic flux density. The graph indicates that ahigher relative magnetic permeability enables easier magnetization of ashielding material when a magnetic field is applied and more effectiveshielding performance.

1. A steel sheet for shielding magnetic field, comprising: by weight %,0.02% or less of carbon (C), 0.001% to 0.05% of silicon (Si), 0.01% to0.2% of manganese (Mn), 0.001% or 0.05% of aluminum (Al), 0.005% or lessniobium (Nb), 0.07% or less of titanium (Ti), 0.01% or less of nitrogen(N), 0.015% or less of phosphorus (P), 0.005% or less of sulfur (S), anda remainder of iron (Fe) and inevitable impurities, wherein amicrostructure has ferrite as main structure, wherein the ferrite has anaverage grain size of 100 μm or greater.
 2. The steel sheet forshielding magnetic field of claim 1, wherein the steel sheet has amaximum size of a precipitate existing at a boundary of or within theferrite grain is 100 nm or less.
 3. The steel sheet for shieldingmagnetic field of claim 1, wherein the steel sheet has a magnetic fluxdensity of 1 T (Tesla) or higher under a magnetic field intensity of 300A/m.
 4. The steel sheet for shielding magnetic field of claim 1, whereinsteel sheet has yield strength of 190 MPa or less, tensile strength of220 MPa to 300 MPa, elongation of 40% or more, and a yield ratio of 0.8or less.
 5. A method for manufacturing a steel sheet for shieldingmagnetic field, comprising: reheating a steel slab comprising by weight%, 0.02% or less of carbon (C), 0.001% to 0.05% of silicon (Si), 0.01%to 0.2% of manganese (Mn), 0.001% or 0.05% of aluminum (Al), 0.005% orless niobium (Nb), 0.07% or less of titanium (Ti), 0.01% or less ofnitrogen (N), 0.015% or less of phosphorus (P), 0.005% or less of sulfur(S), and a remainder of iron (Fe) and inevitable impurities at atemperature of Ac3 to 1250° C.; hot rolling the reheated slab and finishhot rolling at a temperature of Ar3 or higher to obtain a steel sheet;and air cooling the steel sheet to room temperature.
 6. The method formanufacturing a steel sheet for shielding magnetic field of claim 5,further comprising normalizing heat treatment of maintaining at atemperature of Ar3 or higher for at least (1.3t+30) minutes, and then,of furnace cooling or air cooling.
 7. The method for manufacturing asteel sheet for shielding magnetic field of claim 6, further comprisingstress relief annealing of heat treating at a temperature of 800° C. to900° C. for at least (1.3t+30) minutes after the normalizing heattreatment, and then, of furnace cooling.
 8. The method for manufacturinga steel sheet for shielding magnetic field of claim 5, wherein, duringthe hot rolling, rough rolling is performed at a temperature of Tnr to1250° C. before finish hot rolling.